Identification of dopamine D2 receptors in gill of Crassostrea virginica.

The lateral epithelial cells of gill of Crassostrea virginica are innervated by dopamine and serotonin nerves that regulate the beating rate of their lateral cilia. Terminal release of dopamine slows down the beating rate of the cilia, while serotonin release increases the beating rate. Previously, we showed that the dopaminergic, but not the serotonergic, mechanism regulating the beating rate of the lateral cilia was disrupted by manganese treatments and that this disruption was occurring postsynaptically, at the level of the dopamine receptor or further downstream in the signal transduction pathway. In humans manganese toxicity causes Manganism, a neurological disorder with clinical symptoms similar to Parkinson s disease. In this study we utilized pharmacological agents and an immunohistofluorescence technique to characterize the dopamine receptor type present on the lateral ciliated cells of C. virginica gill. Agonists and antagonists to dopamine D1 or dopamine D2 receptors were applied to gill sections and beating rates of the lateral cilia were measured by stroboscopic microscopy. The D2 agonists and D2 antagonists were effective in mimicking or blocking, respectively, the inhibitory actions of dopamine on lateral cilia beating, while application of either D1 agonists or D1 antagonists had no significant effect. In other experiments we used an epilume fluorescence microscopic fitted with FITC filters to view gill sections treated with a primary antibody against D2 receptors and a FITC-linked secondary antibody. Control gill sections without primary antibody exposure were similarly treated and viewed. The D2 antibody treated sections showed bright fluorescent receptor-antibody complexes present at the lateral ciliated cells and other areas of gill, when compared to controls. The results of our immunofluorescence study identify the presence of D2-like receptors on the lateral ciliated cells of C. virginica gill and our pharmacological results indicate that D2-like receptors are the postsynaptic dopamine receptors involved in the cilio-inhibitory response of the lateral cilia. The results of this study, when combined with our previous work, further suggest that the mechanism of action that underlies the dopaminergic neurotoxicity of manganese in gill of C. virginica involves disruption of D2-like receptors. C. virginica continues to provide a simple yet good model with which to study the physiology of dopaminergic systems as well as the pharmacology of drugs affecting biogenic amines.